We have previously shown that the nonvirulent macrophage-tropic molecular infectious clone, SIV/17E-C1, serves as an excellent live attenuated vaccine by inducing immune responses by 8.5 months post-inoculation (p.i.) that protect against challenge with the heterologous primary isolate, SIV/DeltaB670 (Clements et al 1995, J Virol). The nonvirulent phenotype of SIV/17E-Cl, however, is unstable, with 20% of the monkeys infected with this virus developing immunodeficiency disease after a prolonged period of clinical latency. Virulence coincides with the evolution of genetic variants identified by amino acid sequence changes in gp120. To understand how conservative changes created within the attenuated clone permit evasion of immune responses which protect against a more genetically diverse virus strain, monkeys chronically infected with SIV/17E-Cl were further evaluated by a series of intravenous challenges. Six monkeys infected for more than 2 years were selected 4 monkeys with normal CD4+/CD8+ lymphocyte counts, and 2 monkeys whose CD4+ counts had declined after 2 years of infection (monkeys L238 and M689). Monkey L238 had been shown to be previously protected at 8.5 months p.i., from challenge SIV/DeltaB670. Following a CD4+ decline noted in this animal after 18 months p.i., a virus genetically distinct from the parental clone was cultured from the PBMC(SIV/238-var). All six monkeys were challenged with SIV/DeltaB670. Of these, only monkey M689 became infected as evidenced by identification of SIV/DeltaB670 sequences in PBMC DNA and anamnestic antibody response following challenge. The five remaining animals were rechallenged with SIV/239var to determine whether the conservative changes noted for this virus were sufficient to evade an immune response capable of protecting against the more genetically diverse strain, SIV/DeltaB670. Of these, monkey L238 (the animal from which SIV/238var was isolated) responded with anamnestic antibody response. A second monkey (L235), despite the fact that he was asymptomatic at challenge and was protected against challenge with SIV/DeltaB670, became infected with SIV/DeltaB670, became infected with SIV/L238var as evidenced by identification of SIV/238var sequences in PBMC DNA. The 3 remaining monkeys are under evaluation. These experiments provide tools with which to understand the mechanism(s) by which SIV/HIV may evade immune responses which are protective. Further evaluation of the immune responses induced in these animals, and their recognition of epitope(s) present in the variant clone, is crucial to the development of efficacious vaccines.